What are the cause of Black Dot Under Nail ?

A black dot under your nail can be concerning but it's usually nothing to worry about.
The most common cause is a minor injury to the base of the nail. This can happen when you accidentally hit your finger or toe or when you stub your toe.
The trauma causes a blood vessel to break and bleed under the nail.
As the blood dries it turns black and looks like a dot. In most cases the dot will eventually grow out with your nail and disappear on its own.

Skin Cancer

Skin cancer comes in a variety of forms, each named for the type of skin cell from which it develops. Basal cell carcinoma, squamous cell carcinoma, and melanoma are the three most common types of skin cancer.
Acral lentiginous melanoma is a kind of melanoma that causes a black patch under the nail (ALM). Melanomas can develop anywhere on the body, even beneath the fingernails and toenails. Subungual melanoma is a kind of ALM that affects the nail. Subungual melanoma is a rare cancer that affects the big toe or thumb nail. In its early stages, it often appears as a black patch or streak.

A new patch on the skin or a spot that changes in size, shape, or color is the most crucial melanoma warning sign. A dark patch or streak under the nail should be looked up by a dermatologist as soon as possible, especially if the region hasn't been injured recently.
Melanoma is more likely among those who have been exposed to ultraviolet (UV) light for a long time or on a regular basis. In fact, UV light exposure is associated to 90 percent of melanomas. However, some cases occur with little or no exposure, so you should never assume you're safe.

If you've recently hurt the area, the dark patch under the nail could be a subungual hematoma, which is a sort of bruise. Blood gathers under the nail after an injury, causing discolouration.
A toe kick or sports injuries are the most common causes of this type of hematoma. Subungual hematomas are infrequently treated, despite the fact that they are painful.

Nail fungus manifests itself in a variety of ways, but the most common symptom is yellowing of the nails. Some nails turn yellow, brown, or green as time goes on.
You may only notice a spot of discoloration on the nail at first. To safeguard the integrity of the nail and prevent it from spreading to others, a dermatologist should detect and treat nail fungus as away.

Psoriasis of the nails: Psoriasis of the nails can affect both the fingernails and toenails. The nail may move away from the nail bed in some situations, leaving space beneath the nail.
Bacteria can penetrate this area and create a dark green pigment as a result. Melanoma under the nail is frequently confused with this. Fortunately, a dermatologist can provide a variety of psoriasis therapies to aid in the improvement or maintenance of skin integrity.

If you have a black spot under your nail that you are concerned about, you should see a dermatologist right once. Early detection and therapy are critical in boosting the odds of cancer being treated.
Your doctor will want to know when you first noticed a change in your skin, what symptoms you're experiencing, and your medical history.
If your doctor suspects skin cancer, a biopsy will most likely be performed. After numbing the region, the dermatologist will remove all or part of the questionable area.
The tissue is usually submitted to a lab where a dermatopathologist examines it under a microscope.